1. Field of the Invention
The present invention relates to a method for manufacturing a sealed battery, which includes a liquid injection step of injecting an electrolytic solution in a battery container, an introduction step of introducing a detection gas into the battery container, and a leakage detection step of detecting a leakage of the detection gas introduced into the battery container.
2. Description of Related Art
A sealed battery is manufactured in some cases in such a manner that an electrode body formed by laminating or winding a positive electrode, a negative electrode and a separator is housed in a battery container, an electrolytic solution is injected in the battery container, thereafter the battery container is sealed. The electrode body becomes an electric power generating element when the electrolytic solution infiltrates in the electrode body. As a measure of injecting the electrolytic solution in the manufacturing processes of such a sealed battery, there is a technique disclosed in Japanese Patent Application Publication No. 2007-173063 (JP 2007-173063 A) or the like.
According to the technique disclosed in JP 2007-173063 A, an electrolytic solution is injected with an electrolytic solution tank, a syringe, and a liquid injection nozzle. The electrolytic solution tank stores the electrolytic solution and is connected with the syringe through an electrolytic solution feeding pipe. The syringe has an internal space as an electrolytic solution chamber and is connected with the liquid injection nozzle. A joint part of the syringe and the liquid injection nozzle is opened or closed by a liquid injection valve. According to the technique disclosed in JP 2007-173063 A, the electrolytic solution tank is pressurized by nitrogen, and the electrolytic solution is pressure-fed to the electrolytic solution chamber in the syringe. At this time, according to the technique disclosed in JP 2007-173063 A, the liquid injection valve closes between the electrolytic solution chamber and the liquid injection nozzle and the electrolytic solution is filled in the electrolytic solution chamber. Further, according to the technique disclosed in JP 2007-173063 A, the liquid injection valve is opened and the electrolytic solution is injected from the electrolytic solution chamber.
In the manufacturing process of the sealed battery, in order to prevent moisture from intruding into the battery container and degrading battery performance, a leakage detection step of detecting a leakage is performed after the battery container was sealed. In the leakage detection step, for example, the inside of the chamber that houses the battery container is evacuated and a helium amount leaked from a detection region (an internal space of the battery container) per unit time is measured using a helium leakage detector. In the leakage detection step, the measurement result, that is, an output value of the helium leakage detector and a detection threshold value M1 (see FIG. 14) are compared and the sealing property of the battery container is confirmed.
The technique disclosed in JP 2007-173063 A uses nitrogen to pressure feed the electrolytic solution, and accordingly, an abundant amount of nitrogen is present in the internal space of the electrolytic solution tank. Thus, nitrogen mainly dissolves in the electrolytic solution stored in the electrolytic solution tank. In other words, as shown in FIG. 12, when the liquid injection step is performed using the technique disclosed in JP 2007-173063 A, the electrolytic solution containing nitrogen is injected in the battery container.
As shown in FIG. 13, the nitrogen contained in the electrolytic solution diffuses into the detection region when the electrolytic solution infiltrates into the electrode body, and a helium concentration in the detection region is diluted thereby (see N2 shown with a two-dot chain line in FIG. 3). Accordingly, when the electrolytic solution is injected by the technique disclosed in JP 2007-173063 A, after the battery container is sealed, the helium concentration in the detection region decreases due to diffusion of nitrogen. That is to say, according to the technique disclosed in JP 2007-173063 A, the helium concentration of the detection region cannot be maintained until the leakage detection step, as a result thereof, the helium concentration of the detection region during the leakage detection step decreases.
In this case, since a time interval from a time when the battery container is sealed to a time when the leakage detection step is performed varies, the helium concentration in the detection region during the leakage detection step fluctuates (see a graph of the related technique shown in FIG. 11). Therefore, when the helium concentration in the detection region largely decreases due to an influence of the fluctuation of the helium concentration as shown on a graph G11 in FIG. 14, an amount of helium that leaks per unit time decreases, resulting in a small value as a whole.
As is shown in FIG. 14, the detection threshold value M1 of the leakage detection step is set by considering a case where the helium concentration in the detection region is low like this. That is to say, as the detection threshold value M1, an output value of the helium leakage detector is set when a leakage amount of helium per unit time becomes a specified value N in the case where the helium concentration of the detection region during the leakage detection step has the minimum value. Thus, in the case where the helium concentration of the detection region is higher than the minimum value of the helium concentration due to an influence of fluctuation of the helium concentration when a battery container of which leakage amount N0 of helium per unit time is slightly less than the specified amount N is inspected, the output value of the helium leakage detector may exceed the detection threshold value M1 at a relatively high rate (see a point and a graph G12 shown in FIG. 14).
That is to say, in the case where the electrolytic solution was injected using the technique disclosed in JP 2007-173063 A, it was likely that good items were erroneously determined as defective items at a relatively high rate (see a region R11 of erroneous determination shown in FIG. 14). In other words, when the electrolytic solution was injected using the technique disclosed in JP 2007-173063 A, it was likely that the determination rate in the leakage detection step was degraded.